The invention relates to a process for cross-flow filtration with rotating membrane discs, with the filtrate that was passed through the membrane disc(s) being routed in the hollow disc body radially to the rotational axis, and there being led outward through the hollow shaft, with the membrane discs having the same sense of rotation and with a turbulence being created at the membrane surface in the area of overlapping of at least two membrane discs.
Processes and devices are known where liquids are cleaned by fixed membranes such as, for instance, tubular membranes or fixed membrane discs. In the case of membrane discs, the filtrate is fed on one side of the circumference and the concentrate is discharged on the other side. The permeate is discharged after having passed through the membrane. The problem with tubular membranes is that only a small filter area can be used per unit of volume. Also, the filtrate must be backwashed after some time so as to reach the full filtration capacity again. At large filtration capacities of more than 5 to 10 m3 per hour a large number of modules must be placed in parallel or in series to achieve the required filter area. This requires a high amount of pipework and causes enormous space requirement. High pressure losses in the retentate channels (feed of the raw water to be filtered) of the modules necessitate high pump capacities.
For instance, JP 7-75722 (Agency of Ind.) shows a filter having several membrane discs, where the liquid in the disc body is led to the rotational axis and from there outward through the hollow shaft. The disc body seems to consist of a homogenous polypropylene body (without channels). The system works at a vacuum (operating pressure 0.5 kg/cm2). A disadvantage in this case is the large flow resistance in the disc body and in the discharge of the filtrate by vacuum. JP 6-210295 (Hitachi Plant) describes a flocculation and filtration device. This is an open system, in which the filtrate is extracted by vacuum. This is to prevent the risk of clogging. In addition, flocculent and a medium that controls the pH value is injected into the suspension. A circumferential speed of 2.2 m/s is mentioned for the disc. This would cause overflow speeds in the range of approximately 1 m/s for the mentioned operating mode and overlapping. At such a speed, cleaning of the membrane discs would not be sufficient.
The invention is to ensure a filter module with a large filter area per volume unit and thus low space requirement but without the necessity of backwashing.
The invention is characterized by overflow speeds being created at the surface of the membranes, which are in the range between 1 and 5 m/s, and by the concentrate being discharged at an overpressure, where the overpressure may be up to 10 to 14 bar.
At these overflow speeds good cleaning of the membrane surface can be ensured, which prevents or at least minimizes formation of a deposit layer. Due to the overpressure on the concentrate side of the membrane, which may be considerably higher contrary to a vacuum on the permeate side, considerably better filtration can also be achieved.
An advantageous configuration of the invention is characterized by the discs having a rectangular cross section.
A favorable advancement of the invention is characterized by the discs having a triangular cross section.
An advantageous configuration of the invention is characterized by the membrane discs having different rotational speeds. This permits setting a desired overflow speed and/or relative speed in the overlapping area.
An advantageous configuration of the invention is characterized by the permeate being discharged at a vacuum, where the vacuum may be up to 0.5 bar. A suitable pressure difference (trans-membrane pressure) is thereby created for filtration.